Here's my take on your situation, Cheeseburger - give the 5 gallon to the
betta, and let the guppies have the 10. I have 3 male guppies in a 30 gallon long tank, which has other fish but isn't even fully stocked, so plenty of room and hiding places. The guppies are pretty aggressive with each other even with a ton of room - I think it would be way worse in a too-small space. I think less that 10 gallons won't work for a permanent solution.
If it is not possible to give them the 10g, if I were in your shoes, I would be considering rehoming the guppies. How many do you have total in the divided 10g right now?
(If I had a 5 gallon, it would be a puffer tank

)